Coated fasteners with conforming seals

ABSTRACT

A method of installing a fastener into a structure, comprising the steps of providing a fastener having a pin member including an elongated shank having a first end, a second end opposite the first end, a cylindrical shank portion having an outer surface, a head located at the first end of the elongated shank, the head including a bearing surface located on the underside of the head, and a threaded portion located at the second end of the elongated shank and a seal element adapted to be positioned on the pin member such that the seal member is juxtaposed with the bearing surface of the head of the pin member; and installing the fastener into the structure in an installed position.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application relating to and claiming the benefit ofcommonly-owned, co-pending U.S. application Ser. No. 15/059,608 entitled“COATED FASTENERS WITH CONFORMING SEALS,” filed Mar. 3, 2016, which is acontinuation-in-part application relating to and claiming the benefit ofU.S. Provisional Patent Application Ser. No. 62/211,250 entitled“CONFORMING CONICAL SEAL FOR FASTENERS,” filed Aug. 28, 2015, and U.S.application Ser. No. 14/854,223 entitled “FASTENERS WITH COATED ANDTEXTURED PIN MEMBERS,” filed Sep. 15, 2016, which matured into U.S. Pat.No. 9,638,236 issued May 2, 2017, which claims the benefit of U.S.Provisional Patent Application Ser. No. 62/051,602, entitled “FASTENERSWITH COATED AND TEXTURED PIN MEMBERS,” filed Sep. 17, 2014, theentireties of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to fasteners and, more particularly, tofasteners having coated pin members and conforming conical seals.

BACKGROUND OF THE INVENTION

Continuous fiber reinforced composites are extensively used in bothprimary and secondary aircraft components for a variety of applicationswhere light weight, higher strength and corrosion resistance are primaryconcerns. Composites are typically composed of fine carbon fibers thatare oriented at certain directions and surrounded in a supportivepolymer matrix. Since the plies of the composite material are arrangedat a variety of angles, and depending upon the direction of majorloading, the resultant structure is typically a stacked laminatedstructure, which is highly anisotropic and heterogeneous. A significantportion of the composite structure is fabricated as near net-shape, butis drilled in order to facilitate joining of components using mechanicalfasteners. Drilling fastener holes in composite does not compare to theuniformity of aluminum or steel since individual carbon fibers fractureat irregular angles and form microscopic voids between the fastener andthe hole. As the cutting tool wears down, there is an increase ofsurface chipping and an increase in the amount of uncut fibers or resinand delamination. The composite microstructure containing such defectsis referred to as “machining-induced micro texture.”

In addition to their machining challenges, composite structures inaircrafts are more susceptible to lightning damage compared to metallicstructures. Metallic materials, such as aluminum, are very conductiveand are able to dissipate the high currents resulting from a lightningstrike. Carbon fibers are 100 times more resistive than aluminum to theflow of current. Similarly epoxy, which is often used as a matrix inconjunction with carbon fibers, is 1 million times more resistive thanaluminum. The composite structural sections of an aircraft often behavelike anisotropic electrical conductors. Consequently, lightningprotection of a composite structure is more complex, due to theintrinsic high resistance of carbon fibers and epoxy, the multi-layerconstruction, and the anisotropic nature of the structure. Someestimates indicate that, on average, each commercial aircraft in serviceis struck by lightning at least once per year. Aircraft flying in andaround thunderstorms are often subjected to direct lightning strikes aswell as to nearby lightning strikes, which may produce corona andstreamer formations on the aircraft. In such cases, the lightningdischarge typically originates at the aircraft and extends outward fromthe aircraft. While the discharge is occurring, the point of attachmentmoves from the nose of the aircraft and into the various panels thatcompromise the skin of the aircraft. The discharge usually leaves theaircraft structure through the empennage.

The protection of aircraft fuel systems against fuel vapor ignition dueto lightning is even more critical. Since commercial aircraft containrelatively large amounts of fuel and also include very sensitiveelectronic equipment, they are required to comply with a specific set ofrequirements related to the lightning strike protection in order to becertified for operation. It is a well-known fact that fasteners areoften the primary pathways for the conduction of the lightning currentsfrom skin of the aircraft to supporting structures such as spars orribs, and poor electrical contact between the fastener body and theparts of the structure can lead to detrimental fastener arcing orsparking.

To avoid the potential for ignition at the fastener/composite structureinterface, some aircraft use fasteners which are in intimate contactwith the fastener hole. Intimate contact between bare metallic fastenersand the hole in the composite structure has been known to be the bestcondition for electrical current dissipation. One approach to achievefastener-to-composite hole intimacy is to use a sleeved fastener. Thisapproach involves first inserting a close fitting sleeve in the hole. Aninterference-fit pin is then pulled into the sleeve. This expands thesleeve to bring it in contact with the wall of the hole in the compositestructure. Although the sleeve substantially reduces the gap between thefastener and composite structure, it cannot eliminate the small gapscreated due to the presence of drilling induced texture across thecomposite inner-hole surface. This machining induced texture alsoentraps excess sealant, an insulating material, inhibiting the intimatecontact between the sleeve and the hole. This situation becomes evenworse as the cutting tool wears, resulting in more and larger machininginduced defects.

In order to avoid this condition, the current must dissipate through thecarbon fibers exposed along the inner surface of the fastener hole. Ifthe fastener is not in intimate contact with the inside of the hole, theinstantaneous joule energy driven by the lightning strike leads toplasma formation within the gap that leads to air/metal vapor ionizationwhich leads to pressure buildup that blows out in the form of a spark orhot particle ejection. The intrinsic high conductivity of metallicfasteners and the large number of fasteners used in aircraftconstruction combine to create a condition of a high probability oflightning attachment to fasteners.

SUMMARY OF THE INVENTION

In an embodiment, a method of making a fastener, comprising the stepsof: providing a pin member including an elongated shank having a firstend, a second end opposite the first end, a cylindrical shank portionhaving an outer surface, a head located at the first end of theelongated shank, the head including a bearing surface located on theunderside of the head, and a threaded portion located at the second endof the elongated shank; and attaching a seal element to the pin memberin a position that is juxtaposed with the bearing surface of the head ofthe pin member. In an embodiment, the method includes the step ofcoating at least a portion of the pin member with a coating. In anembodiment, the coating is a metallic coating. In an embodiment, themetallic coating is selected from the group consisting of gold, silver,and copper. In an embodiment, the coating is made from a material havingan electrical conductivity higher than 20% IACS. In an embodiment, thecoating step includes coating the head of the pin member with thecoating. In an embodiment, the coating step includes coating the outersurface of the cylindrical shank portion with the coating. In anembodiment, the coating step includes coating the head of the pin memberand the cylindrical shank portion with the coating. In an embodiment,the coating step includes coating the threaded portion and thecylindrical shank portion of the pin member with the coating. In anembodiment, the coating step includes coating the pin member fully withthe coating.

In an embodiment, a method of installing a fastener into a structure,comprising the steps of: providing a fastener having a pin memberincluding an elongated shank having a first end, a second end oppositethe first end, a cylindrical shank portion having an outer surface, ahead located at the first end of the elongated shank, the head includinga bearing surface located on the underside of the head, and a threadedportion located at the second end of the elongated shank and a sealelement adapted to be positioned on the pin member such that the sealmember is juxtaposed with the bearing surface of the head of the pinmember; and installing the fastener into the structure in an installedposition. In an embodiment, the method includes the step of coating atleast a portion of the pin member with a coating. In an embodiment, thecoating is a metallic coating. In an embodiment, the metallic coating isselected from the group consisting of gold, silver, and copper. In anembodiment, the coating is made from a material having an electricalconductivity higher than 20% IACS. In an embodiment, the coating stepincludes coating the head of the pin member with the coating. In anembodiment, the coating step includes coating the outer surface of thecylindrical shank portion with the coating. In an embodiment, thecoating step includes coating the head and the cylindrical shank portionof the pin member with the coating. In an embodiment, the coating stepincludes coating the cylindrical shank portion and the threaded portionof the pin member with the coating. In an embodiment, the coating stepincludes coating the pin member fully with the coating. In anembodiment, the structure includes a composite material. In anembodiment, the structure is substantially made from the compositematerial. In an embodiment, the structure is partially made from thecomposite material. In an embodiment, the structure includes a metallicmaterial. In an embodiment, the metallic material is aluminum. In anembodiment, the structure is made substantially from the metallicmaterial. In an embodiment, the structure is made partially from themetallic material. In an embodiment, the method further comprises thestep of trimming the seal element flush with the structure. In anembodiment, the trimming step includes sanding the seal element. In anembodiment, the method further comprises the step of providing ametallic mesh on an outer surface of the structure, wherein when thefastener is in its installed position, the sealing element of thefastener is in direct physical and electrical contact with the metallicmesh. In an embodiment, the seal element includes a sealing portionhaving a first side and a second side opposite the first side, a lipextending from the first side of the sealing portion, the lip being indirect physical and electrical contact with the metallic mesh. In anembodiment, the metallic mesh is made from copper.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of an embodiment of a pin memberhaving selected surfaces coated with a material;

FIG. 2 is a bottom perspective view of an embodiment of a seal;

FIG. 3 is a bottom perspective view of the pin member and the seal shownin FIGS. 1 and 2, respectively, assembled together;

FIG. 4 is a photograph of an embodiment of an outer surface of thecoated pin member shown in FIG. 1;

FIG. 5 is a photograph of the topography of an outer surface of anembodiment of the coated pin member shown in FIG. 1;

FIGS. 6 and 7 are photographs of an embodiment of a pin member having atextured surface;

FIG. 8 is a side elevational view of an embodiment of a pin memberhaving a conforming seal element;

FIGS. 9A and 9B are top plan and side views, respectively, of anembodiment of a conforming seal element;

FIG. 10 illustrates a screenshot of a stress distribution analysis of aninstalled fastener;

FIG. 11 is a micro-photograph of the cross-section of a standardfastener installed in a structure;

FIG. 12A is a micro-photograph that illustrates a standard fastenerinstalled in a structure, while FIG. 12B is a micro-photograph thatillustrates the pin member and the seal element shown in FIG. 8installed in a structure;

FIGS. 13A and 13B show pre-sanding and post-sanding steps of a structurecontaining a fastener of FIG. 8 installed therein;

FIGS. 14A and 14B are schematic illustrations of the fastener of FIG. 8before and after a sanding step, respectively;

FIG. 15A is a micro-photograph of a standard fastener installed in astructure with an associated copper mesh, while FIG. 15B is amicro-photograph of a fastener shown in FIG. 8 installed in a structurewith an associated copper mesh;

FIG. 15C is graph and associated photographs corresponding to specificdata points on the graph showing flushness tolerance between thefastener shown in FIG. 8 and a standard fastener;

FIGS. 16A and 16B are micro-photographs of a conventional fastenerinstalled in a structure (40 times and 600 times magnification,respectively), while FIGS. 16C and 16D are micro-photographs of afastener as shown in FIG. 8 installed in a structure (25 times and 1000times magnification, respectively);

FIG. 17A is a photograph showing the effects of lightning damage on astandard fastener installed in a structure, while FIG. 17B is aphotograph showing the effects of lightning damage on a fastener asshown in FIG. 8 installed in a structure;

FIG. 17C is a micro-photograph showing the effects of lightning damageon a standard fastener installed in a structure, while FIG. 17D is amicro-photograph showing the effects of lightning damage on a fasteneras shown in FIG. 8 installed in a structure;

FIG. 18A through 18F illustrate a series of simulation results showingreduction of contact resistance and optimized electrical intimacy of thefastener of FIG. 8; and

FIG. 19 is a graph showing electric contact resistivity versus preloadforce between the fastener shown in FIG. 8, a fastener with a coated pinmember, and an anodized fastener.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, in an embodiment, a pin member 12 includes anelongated shank 14 having a cylindrical shank portion 16, a head 18 atone end of the cylindrical shank portion 16 and a threaded portion 20 atan opposite end of the cylindrical shank portion 16. In an embodiment,the head 18 is a countersunk head. In an embodiment, the outer surfacesof the head 18, including an underside surface (e.g., bearing surface)21 of the head 18, and the cylindrical shank portion 16 are coated withcoating 22. In an embodiment, the coating 22 is tungsten. In anotherembodiment, the coating 22 is molybdenum. In another embodiment, thecoating 22 is a refractory metal, such as tantalum or niobium. Inanother embodiment, the coating 22 is a refractory ceramic, such asalumina (Al2O3), silica (SiO2) or other metal oxides. In anotherembodiment, only the outer surfaces of the head 18 are coated with thecoating 22. In another embodiment, only the outer surface of thecylindrical shank portion 16 is coated with the coating 22. In anembodiment, the coating 22 lowers electrical contact resistance andreduces probability of arc initiation/damage. In an embodiment, thecoating 22 includes a high electrical conductivity (higher than 20%IACS) and be galvanically compatible to a structure (e.g., anodic indexless than 1.0V) for corrosion resistance. In an embodiment, thestructure includes a composite structure. In another embodiment, thestructure includes a metal structure. In another embodiment, thestructure includes a fiber metal laminate structure.

In an embodiment, the coating 22 is a thin film coating having athickness in a range of about one (1) nanometer to about two-hundred(200) microns. In an embodiment, the coating 22 is applied by physicalvapor deposition. In another embodiment, the coating 22 is applied bychemical vapor deposition. In another embodiment, the coating 22 isapplied by a selective additive process. In another embodiment, thecoating 22 is applied by electroplating. In another embodiment, thecoating 22 is applied by a spraying process. In another embodiment, thecoating 22 is applied by cold spraying. In another embodiment, thecoating 22 is applied by thermal spraying. In another embodiment, thecoating 22 is applied by plasma coating. In another embodiment, thecoating 22 is applied by a sputter deposition process.

In another embodiment, the outer surfaces of the head 18 and thecylindrical shank portion 16 are textured. In an embodiment, the outersurfaces of the head 18 and the cylindrical shank portion 16 of the pinmember 12 are textured to conform to the machine-induced micro textureinherent in fastener holes drilled in composite structures, and toprovide mechanical interlocking between the pin member 12 and thecomposite structure. In an embodiment, the textured pin member 12excavates excess entrapped sealant during installation of the fastenerwhile bringing the fastener in intimate contact with the structure, and,thus, lowering the electrical contact resistance at the interface. Theterm “intimate contact” as used herein means that the textured outersurface of the pin member 12 is deformed into all or substantially allof voids between the pin member and the composite structure. In anotherembodiment, only the outer surfaces of the head 18 are textured. Inanother embodiment, only the outer surface of the cylindrical shankportion 16 is textured.

In an embodiment, the textured surfaces of the pin member 12 are createdby surface reshaping processes, such as media blasting. In anembodiment, the textured surfaces of the pin member 12 are grit blasted.In an embodiment, the grit blasting utilizes fine grit glass bead media(100-170 mesh). In an embodiment, the grit blasting is performed untilthere is full coverage of the outer surfaces of the pin member 12 to betextured. In an embodiment, the grit blasting is performed for at leastone minute. In another embodiment, the grit blasting is performed forabout one minute. In an embodiment, the grit blasting step is performedtwice. In another embodiment, the textured surfaces of the pin member 12are created by removal processes, such as selective electro-etching,laser etching, abrasive blasting, and mechanical polishing. In anotherembodiment, the textured surfaces of the pin member 12 are created bychemical etching. In an embodiment, the chemical etching utilizes 50/50hydrochloric acid (HCl). In an embodiment, the chemical etching isperformed for about 30 minutes. In an embodiment, the pin member 12 isrinsed with distilled water for about 15-20 seconds, and dried withforced, room-temperature air for approximately 1 to 2 minutes.

In another embodiment, the surfaces of the head 18 and the cylindricalshank portion 16 of the pin member 12 are coated and textured by acombination of a coating process and a texturing process as describedabove. In an embodiment, a combination of the coating and texturingprocesses can be used to develop functional characteristics of the pinmember 12, based on a specific loading pattern thereof. For example, inan embodiment, where the preload is high, the texturing/coating isperformed to lower contact resistance. At locations which carry nopreload and are not in intimate contact with the composite layer,mitigation of plasma generation and arc formation/damage is desired.

In an embodiment, the pin member 12 is part of a fastener that isadapted to secure a plurality of work pieces of to one another, and isadapted to be installed within aligned holes in such work pieces. In anembodiment, the work pieces are made of a composite material. In anotherembodiment, the work pieces are made of metal. In another embodiment,the work pieces are made from a fiber metal laminate. In an embodiment,the fastener includes a locking member (not shown in the Figures). In anembodiment, the locking member is a nut. In another embodiment, thelocking member is a collar. In an embodiment, a fastener 10 includes thepin member 12 and a seal 24 installed on the bearing surface 21 of thehead 18 of the pin member 12, as shown in FIGS. 2 and 3, and to bediscussed in further detail below.

During a lightning strike on an aircraft, the lightning typicallyattaches to the head 18 of the pin member 12 first. In an embodiment,the coated and/or textured pin member 12 improves contact resistance. Inthis regard, all solid surfaces are rough on a micro-scale and contactbetween two engineering bodies occurs at discrete spots produced by themechanical contact of asperities on the two surfaces. For all solidmaterials, the true area of contact is a small fraction of the apparentcontact area. Electrical current lines get increasingly distorted as thecontact spot is approached and flow lines bundle together to passthrough “a-spots”. An electrical junction consists of a number ofcontact “a-spots” through which electrical current passes from oneconnector component to the other and is often characterized byelectrical contact resistance of the interface.

When a fastener is installed in a composite structure using a clearancefit, the primary load bearing surface of the pin member 12 as installedis the bearing surface 21 of the head 18. This is an electrical contactthrough which it is desired to pass a high frequency, high voltagecurrent and is a significant first line of defense to the lightningstrike. If the current has a path to flow easily, no arcing andresultant damage would occur. The pin or bolt to composite interface canprove to be an inefficient electrical contact due to dissimilarmaterials, presence of electrically insulating films like aircraftsealant and/or hard oxide layers on the surface and irregular cutpattern of the composite. To allow current to flow easily through thepin/bolt to composite interface, the interface contact resistance isdesired to be low.

Contact resistance is highly dependent on the applied load on both thesurfaces that brings them in contact and electrical and mechanicalproperties of the material surface in contact. A soft material at theinterface with high electrical conductivity lowers the contactresistance, as do higher loads. The load in a pin member joint isprovided by the preload and is primarily geometry/design dependent. Asdescribed above, the material coating 22 or texturing on the bearingsurface 21 of the head 18 is used to both provide a low resistivitymaterial at the contact interface and a soft conforming layer for bettercontact with the structure. Soft materials with high electricalconductivity, such as copper, gold, silver or other metals/materials canbe used to lower contact resistance (see, e.g., the copper seal 24 shownin FIGS. 2 and 3).

The surfaces of the pin member 12, as described above, can also betextured to enable better intimacy with the surrounding composite layer.As the textured pin member 12 is installed, the textured pin memberdeforms into the small voids that are created during drilling of thecomposite layer. As the textured surfaces deform into the voids, theydisplace the entrapped sealant during fastener installation. Theinsertion of the pin member 12 causes the excess sealant to be extrudedoutside the pin member 12/composite interface. Thus, the textured pinmember 12 excavates excess entrapped sealant during installation of thefastener while bringing the pin member 12 in intimate contact with thecomposite structure. The finish texture of the pin member's 12 surfacesis adjusted to provide a surface micro-roughness (Sa) value in order toincrease the level of conformity and mechanical interlocking. In anembodiment, the surface roughness (Sa) is greater than 0.5 micron.

As described above, FIG. 1 shows an embodiment of a tungsten coated pinmember 12. In an embodiment, plasma coating was used to deposit tungstenon the pin member 12 and achieve a surface roughness (Sa) equal orgreater than 7 micron. FIG. 2 shows the seal 24 and FIG. 3 shows the pinmember 12 with the seal 24 installed on it to promote intimacy with thecomposite layer on the bearing surface 21 of the head 18. In anembodiment, the seal 24 is frusto-conical in shape, and is sized andshaped to fit on the bearing surface 21 of the head 18. In anotherembodiment, this can also be achieved by copper coating the bearingsurface 21 of the head 18. In another embodiment, the seal 24 is acaptive washer. In another embodiment, the seal 24 is coated with acoating. In an embodiment, the coating of the seal 24 includes thecoating 22.

FIG. 4 shows a photograph of the texture variation of the coated pinmember 12, while FIG. 5 shows the surface topography of the coated pinmember 12. In an embodiment, the coated surfaces of the pin member 12have an average surface roughness (Sa) of 7.5 micron. FIGS. 6 and 7 arephotographs of the textured pin member 12 at 40× and 190× magnification,respectively. As can be seen in FIGS. 6 and 7, the textured pin member12 exhibits a substantially rough finish. In an embodiment, the texturedpin member 12 provides improved electrical contact along the texturedsurfaces of the pin member 12, which minimizes the dielectric effectcaused by the sealant, promotes easier transfer of electric current,reduces the voltage potential across the pin member 12/compositeinterface, and thus enables transfer of electric current without anybreakdown effects like arcing.

In an embodiment, in a clearance fit hole, there is no preload betweenthe shank 14 of the pin member 12 and the composite layer, and thuselectrical contact is relatively poor. Thus, it would be difficult toensure significant current flow between the pin member 12 and thecomposite layer. In case sufficient currents are not conducted by thebearing surface 21 of the head 18, there would be a possibility ofarcing at the gap between the shank 14 and the adjacent compositelayers. Arc formation under such conditions typically initiates in themetal vapor itself. The presence of a high temperature melting materialwith high conductivity will ensure that sufficient metal vapor is notpresent to initiate arcing. Even if arcing is initiated, the volume ofplasma will be low. Higher conductivity will also ensure that current ismore easily passed between the shank 14 and composite layer if contactis available. As described above, in certain embodiments, materials liketungsten, molybdenum, or refractory metals/ceramics can be used as thecoating 22 on the shank 14 of the pin member 12 to ensure reduction inarc damage. Since lightning strikes generate high frequency currents,current would typically flow close to the fastener surface due to “skineffect”. The coating on the pin member 12 also helps in this respectthat a higher temperature melting point and high conductivity materialwould carry most of the current lowering the likelihood of fastenermelting or plasma generation.

Thus, the coated/textured pin member 12:

-   -   Improves electrical contact between composite and fastener        surface;    -   Minimizes fastener arcing during lightning strikes;    -   Provides gap filling and mechanical interlocking capabilities;    -   Reduces likelihood of plasma formation during arcing around the        fastener shank;    -   In case arcing occurs in the fastener, reduces the volume of        plasma generated to make it easier to be contained.        Coated Fasteners with Conforming Conical Seals

Referring to FIGS. 8, 9A and 9B, in an embodiment, a fastener 110includes a pin member 112 having an elongated shank portion 114 with asmooth cylindrical shank portion 115, a head 116 at one end of thesmooth cylindrical shank portion 115 and a threaded portion 117 at anopposite end of the smooth cylindrical shank portion 115. In anembodiment, the head 116 is a countersunk head. In an embodiment, alocking member is adapted to be installed to the pin member 112 (notshown in the Figures). In an embodiment, the locking member is athreaded nut that engages the threaded portion 117 of the pin member112. In another embodiment, the locking member is a collar adapted to beswaged into the lock grooves of the threaded portion 117 of the pinmember 112.

In an embodiment, the pin member 112 is fully coated with a coating 119.In an embodiment, the coating 119 is a metallic coating. In anembodiment, the coating 119 is a soft, metallic coating. That is, thecoating 119 is applied to the elongated shank portion 114, including thesmooth cylindrical shank portion 115 and the threaded portion 117, andthe head 116, including an underside (e.g., bearing surface 120) of thehead 116. In an embodiment, the coating 119 is copper. In anotherembodiment, the coating 119 is silver. In another embodiment, thecoating 119 is gold. In other embodiments, the coating 119 is made froma material having a high electrical conductivity, for example, amaterial having an electrical conductivity higher than 20% IACS.

In other embodiments, the coating 119 can consist of any one of thecoatings 22 with respect to the embodiment of the pin member 12, whichare described in detail above.

In another embodiment, the pin member 112 is partially coated with thecoating 119. In an embodiment, the coating 119 is applied to the head116, including the underside 120 of the head 116, of the pin member 116.In another embodiment, the coating 119 is applied to the head 116(including the underside 120 of the head 116) and to the smoothcylindrical shank portion 115 of the pin member 112. In anotherembodiment, the coating 119 is applied to the smooth cylindrical shankportion 115 of the pin member 112. In another embodiment, the coating119 is applied to the smooth cylindrical shank portion 115 and thethreaded portion 117 of the pin member 112.

In another embodiment, the pin member 112 does not include the coating119.

Still referring to FIGS. 8, 9A and 9B, in an embodiment, a conformingseal element 118 is attached to the elongated shank portion 114 andjuxtaposed with the bearing surface 120 of the head 116 of the pinmember 112. In an embodiment, the seal element 118 is separate anddistinct from the pin member 112. In an embodiment, the seal element 118can be positioned within a hole of a structure and the pin member 112can then be inserted into the seal element 118 during installation ofthe fastener 110. In an embodiment, the seal element 118 isfrusto-conical in shape and includes a centrally located,circular-shaped aperture 122 that is sized and shaped to fit around theshank portion 114 of the pin member 112 and juxtaposed with the bearingsurface 120 of the head 116 of the pin member 112. In an embodiment, theseal element 118 includes a sealing portion 121. In another embodiment,a lip 123 extends from one side of the sealing portion 121. In anembodiment, the lip 123 is angled upwardly from the sealing portion 121.In another embodiment, a tubular portion 125 extends axially from anopposite side of the sealing portion 121. In an embodiment, the sealelement 118 is made from copper. In an embodiment, the sealing portion121 of the seal element 118 has a thickness in a range of about 5microns to about 100 microns.

It is noted that all solid surfaces of the pin member 112 and astructure 150 in which the fastener 110 is adapted to be installed arerough on a microscopic scale. Surface micro-roughness consist of peaksand troughs whose shape, variations in height, average separation andother geometric characteristics depend on the details of the processused to generate the surfaces. Contact between two engineering bodiesoccurs at discrete microscopic spots that are the result of mechanicalcontact of asperities on the two surfaces. For all solid materials, thearea of true contact is a small fraction of the nominal contact area fora wide range of normal contact loads.

Referring to FIGS. 10 and 11, when mechanical load is exerted throughthis contact area, the mode of deformation of contact asperities iselastic, plastic, or a mixture of plastic and elastic depending on thelocal mechanical stresses, and on the properties of the material such asthe elastic modulus and hardness. In a bulk electrical interface wherethe mating components are metals, the contacting surfaces often containan oxide or other electrical insulating layers. Generally the interfacebecomes electrically conductive only when electrically insulation filmsare displaced at the asperities of the contacting surfaces or thepotential across the interface exceeds the dielectric strength of theelectrically insulation film. For the sake of simplicity in the field ofelectrical connectors, the discrete spots are often assumed to becircular. This assumption provides an acceptable geometric descriptionof the average contact spots where the roughness topographies of themating surfaces are isotropic. While this assumption is acceptable formetallic structures, it becomes invalid when the mating surfaces arecharacterized by directional micro-texture or are clearly anisotropic innature. The true area of contact between a fastener and the surroundingCFRP structure is a very small percent of the nominal area due to themulti-layered construction and anisotropic nature of CFRP structures,which further complicates quality of the electrical contact between thefastener and surrounding CRFP structure.

FIG. 12A illustrates a standard fastener installed in a structure (e.g.,an aluminum panel), which shows microgaps between the head of a pinmember and the structure. In an embodiment, with reference to FIG. 12B,the conforming seal element 118 is adapted to maximize the true area ofcontact between the fastener (e.g., the bearing surface 120 of the head116 of the pin member 112) and a structure 150 with minimum mechanicalload. In an embodiment, the structure 150 includes a composite material.In another embodiment, the structure 150 is substantially made from acomposite material. In another embodiment, the structure 150 ispartially made from a composite material. In another embodiment, thestructure 150 includes a metallic material. In an embodiment, themetallic material is aluminum. In another embodiment, the structure 150is made substantially from a metallic material. In another embodiment,the structure 150 is made partially from a metallic material.

In an embodiment, the conforming seal element 118 includes a multi-layerconstruction with a relatively soft, yet highly electrically conductivebase layer, which provides macroscopic conformity, and a softer toplayer, which provides microscopic conformity.

In an embodiment, a method by which the fastener 110 with the sealelement is installed is described hereinbelow. In an embodiment, withreference to FIGS. 8, 9A, 9B, 13A, 13B, 14A and 14B, the method includesthe steps of coating the pin member 112 with the coating 119 (eitherfully or partially as described above), attaching the seal element 118to the fastener 110 (e.g., the pin member 112), and installing thefastener 110 in the structure 150. In an embodiment, the coating step isnot included when the pin member 112 is not coated with the coating 119as described above. In another embodiment, the seal element 118 can bepositioned within a hole of the structure 150 and the pin member 112 canthen be inserted into the seal element 118 during installation of thefastener 110. In an embodiment, with respect to the installation step, apreload to the fastener 110 is provided by the locking member (e.g., nutor collar), and a force is exerted on the structure 150 by the pinmember 112 with the seal element 118 positioned between the head 116 ofthe pin member 112 and the structure 150. As the seal element 118conforms to the inherent micro-roughness between the head 116 of the pinmember 112 and the structure 150, a portion of the seal element 118 isextruded upward the edge of the pin member 112 and protrudes above thesurface of the structure 150. With reference to FIGS. 13A, 14B, 14A and14B, the seal element 118 is trimmed flush with the surface of thestructure 150 by sanding the top of the seal element 118 (e.g.,proximate to the lip 123) and, if necessary, the structure 150. In anembodiment, the sanding step is simultaneous with the preparation of thesurface of the structure 150 for the application of paint 152.

FIGS. 15A and 15B are photographs illustrating the cross-sections of apin member without the seal element 118 (FIG. 15A) and the pin member112 with the seal element 118 (FIG. 15B). As shown, the inclusion of theseal element 118 is provided along with a copper mesh 154 and improvespaint adhesion.

Referring to FIG. 15C, the fastener 110 improves a range of countersinkwithin the structure 150 over which the connection with the copper mesh152 is maintained. As seen on the graph shown in FIG. 20, the flushnesstolerance of the fastener 110, shown on the left, is wider than abaseline fastener without the seal element, as shown on the right.

FIGS. 16A through 16D are photographs showing the difference in pin/CFRPinterface between a conventional fastener without a seal element (FIGS.16A and 16B) and a fastener with the seal element 18 (FIGS. 16C and16D). Micro-level conformance between the seal element 118 and the CFRPstructure 50 enhances the current transfer from fastener to thestructure 150 and reduce arcing.

FIGS. 17A through 17D illustrate the differences in the effects ofdamage in aluminum panels of a fastener without the seal element 118(FIGS. 17A and 17C) and a fastener with the seal element 118 (FIGS. 17Band 17D). The seal element 118 increases the electrical intimacy betweenthe fastener 110 and the structure in the area adjacent to the sealelement 118. As will be described in more detail below, this reduces themagnitude of the electric field near the locking member (e.g., nut orcollar).

With reference to FIGS. 18A through 18F, which illustrate simulationresults, the seal element 118 reduces contact resistance around the head116 of the pin member 112 and results in optimized electrical intimacy.This nanoscale conformity leads to improved current transfer into theupper panel of the aircraft structure 150. During a lightning strike,the external discharge, which attaches to the head 116, will tend toattach to regions having larger electric fields. In the case of thefastener 110 having the seal element 118, the electric fields are muchlower, resulting in so-called equipotential surfaces with a flatterfield profile. This field flattening effect minimizes the amount ofstructural damage caused by large concentrated flows through sharpedges.

An advantage of the seal element 118 is the large reduction of thecharge buildup between the fastener 110 and surrounding materials withinthe fastener assembly. The time dependent electric potential has a lowerpeak value, which results in a large reduction of the electric fieldmagnitudes around the bearing surface of the nut region. Typically,large fields around the nut region and sharp edges can result indielectric breakdown and edge glow phenomenon. The large reduction inelectric fields is a direct result of the enhanced current transport.

Referring to FIG. 19, the fastener 110 includes a reduced contactresistance. Contact resistivity measurements show current transferimprovement with the fastener 110 having the coating 119 and the sealelement 118 over baseline pin members without the coating 119 and theseal element 118.

It should be understood that the embodiments described herein are merelyexemplary and that a person skilled in the art may make many variationsand modifications without departing from the spirit and scope of theinvention. All such variations and modifications are intended to beincluded within the scope of the claims.

What is claimed is:
 1. A method of making a fastener, comprising thesteps of: providing a pin member including an elongated shank having afirst end, a second end opposite the first end, a cylindrical shankportion having an outer surface, a head located at the first end of theelongated shank, the head including a bearing surface located on theunderside of the head, and a threaded portion located at the second endof the elongated shank; and attaching a seal element to the pin memberin a position that is juxtaposed with the bearing surface of the head ofthe pin member.
 2. The method of claim 1, further including the step ofcoating at least a portion of the pin member with a coating.
 3. Themethod of claim 2, wherein the coating is a metallic coating.
 4. Themethod of claim 3, wherein the metallic coating is selected from thegroup consisting of gold, silver, and copper.
 5. The method of claim 2,wherein the coating is made from a material having an electricalconductivity higher than 20% IACS.
 6. The method of claim 2, wherein thecoating step includes coating the head of the pin member with thecoating.
 7. The method of claim 2, wherein the coating step includescoating the outer surface of the cylindrical shank portion with thecoating.
 8. The method of claim 7, wherein the coating step includescoating the head of the pin member with the coating.
 9. The method ofclaim 7, wherein the coating step includes coating the threaded portionof the pin member with the coating.
 10. The method of claim 2, whereinthe coating step includes coating the pin member fully with the coating.11. A method of installing a fastener into a structure, comprising thesteps of: providing a fastener having a pin member including anelongated shank having a first end, a second end opposite the first end,a cylindrical shank portion having an outer surface, a head located atthe first end of the elongated shank, the head including a bearingsurface located on the underside of the head, and a threaded portionlocated at the second end of the elongated shank and a seal elementadapted to be positioned on the pin member such that the seal member isjuxtaposed with the bearing surface of the head of the pin member; andinstalling the fastener into the structure in an installed position. 12.The method of claim 11, further including the step of coating at least aportion of the pin member with a coating.
 13. The method of claim 12,wherein the coating is a metallic coating.
 14. The method of claim 13,wherein the metallic coating is selected from the group consisting ofgold, silver, and copper.
 15. The method of claim 12, wherein thecoating is made from a material having an electrical conductivity higherthan 20% IACS.
 16. The method of claim 12, wherein the coating stepincludes coating the head of the pin member with the coating.
 17. Themethod of claim 12, wherein the coating step includes coating the outersurface of the cylindrical shank portion with the coating.
 18. Themethod of claim 17, wherein the coating step includes coating the headof the pin member with the coating.
 19. The method of claim 17, whereinthe coating step includes coating the threaded portion of the pin memberwith the coating.
 20. The method of claim 13, wherein the coating stepincludes coating the pin member fully with the coating.
 21. The methodof claim 11, wherein the structure includes a composite material. 22.The method of claim 21, wherein the structure is substantially made fromthe composite material.
 23. The method of claim 21, wherein thestructure is partially made from the composite material.
 24. The methodof claim 11, wherein the structure includes a metallic material.
 25. Themethod of claim 24, wherein the metallic material is aluminum.
 26. Themethod of claim 24, wherein the structure is made substantially from themetallic material.
 27. The method of claim 24, wherein the structure ismade partially from the metallic material.
 28. The method of claim 11,further comprising the step of trimming the seal element flush with thestructure.
 29. The method of claim 28, wherein the trimming stepincludes sanding the seal element.
 30. The method of claim 28, furthercomprising the step of providing a metallic mesh on an outer surface ofthe structure, wherein when the fastener is in its installed position,the sealing element of the fastener is in direct physical and electricalcontact with the metallic mesh.
 31. The method of claim 30, wherein theseal element includes a sealing portion having a first side and a secondside opposite the first side, a lip extending from the first side of thesealing portion, the lip being in direct physical and electrical contactwith the metallic mesh.
 32. The method of claim 31, wherein the metallicmesh is made from copper.